高超平板边界层转捩的气动光学诊断技术

Aero-optical diagnostic technique for hypersonic boundary layer transition on a flat plate

高超声速平板边界层转捩是基础研究和工程应用中的一个重要的难题,对于转捩区域的理论估计和试验诊断与实际飞行试验结果有较大的偏差。随着光学测试技术的发展,高超声速气动光学研究水平不断提升,在气动光学中,一个重要的问题是边界层流动对穿过流场的光束存在影响,使光束发生畸变,包括光束的偏折、模糊和抖动。将上述两个问题结合起来考虑,高超声速边界层的转捩过程会使穿过边界层的光束的畸变情况发生变化,通过研究流场状态变化与光束畸变的变化之间的关联,尝试建立一种以气动光学测试方法为基础的边界层转捩诊断技术。本文具体分析了高超平板边界层转捩与细光束抖动特性之间的联系,提出了可以通过气动光学的细光束技术和高速图像拍摄技术对高超平板边界层转捩进行检测识别的新思路,并在FD-20风洞中来流Ma=8,ReL=1×107/m条件下,采用SABT(Small-Aperture Beam Technique,即细光束技术)以及HICS(High-speed Imaging Camera System,即高速图像拍摄系统),开展了平板模型边界层转捩的细光束抖动特性实验。试验模型的中心子午线上布置了有机玻璃窗口,可通过窗口测量光束通过测点及边界层后的抖动情况,结果表明再边界层的不同流向位置,细光束的抖动出现了幅值方差的变化。将这一试验结果与同一模型中心子午线的气动热测试结果相比较表明,细光束抖动特性的变化趋势与转捩区中的热流增长趋势相同,因此,采用上述气动光学检测技术对高超平板边界层转捩进行诊断和识别,存在可行性。

Hypersonic boundary-layer transition on a flat plate is fundamental research problem for both theoretical investigations and engineering applications,however,evaluations and experimental estimations of the transition region have large discrepancies with the results n the real flight.With the development of the optical measurement techniques,the investigation level of the hypersonic aero-optics is gradually improved.In aero-optics,the effect of the boundary-layer flow to the light beam through the flow field,that is the distortions of the light beam tncluding deflection,blur and jitter,is one of the important problems.Considering the above mentioned two problems,the hypersonic boundary-layer transition could result in the change of the distortion of the light beam,so that one could investigate the linkage between those two phenomena to built up one boundary-layer transition diagnostic technique based on the aerooptical measurement method.The linkage between the hypersonic boundary layer transition on a flat plate and the jittering characteristics of the small-aperture beam(SAH)is theoretically analyzed.Due to the concept of transitional aero-optics,a new idea which could diagnose the hypersonic boundary layer transition on a flat plate using SAB technique and HICS(High-speedImaging Camera System)is suggested,and is preliminarily validated by those in FD-20gun tunnel with the Mach number of8and the unit Reynolds number m.To investigate the effects of the boundary-layer transition on a flat plate to the jittering characteristics of the small-aperture beam,three polymethyl methacrylate windows are arranged in the centerline of the test model.The jittering of the small-aperture beam through the windows could be measured and the results validate the assumption that the standard deviation of the jittering data could be changed at the different streamwise locations in the boundary layer.are compared with the heat transfer measurements on the centerline of another same model,same trend between those two experimental results in the transitional region of the hypersonic boundary layer is found.Thus?it is feasible to diagnose the transition of the hypersonic boundary layer by those aero-optical techniques.